1. Field of the Invention (Technical Field)
The present invention relates to enhancing linear and nonlinear optical emission using nanoparticles, wherein the nanoparticles are either non-aggregated or aggregated, and microcavities. The aggregrated nanoparticles comprise fractals. Microcavities are used in combination with nanoparticles for greatly enhanced optical emission.
The present invention also relates to optical methods and structures employing semicontinuous metal films and microresonator/semicontinuous-metal-film composites.
2. Background Art
Recently, fractal aggregates of gold, silver, and other noble metals have received attention in the field of linear and nonlinear optical research. Fractals comprise aggregates of particles in colloidal solutions, sols and gels, and soot and smoke. Also, most macromolecules exist in the form of fractals. A fractal aggregate is a system of interacting particles, with special scale-invariant geometry. Scale-invariance in particle aggregates manifests itself in spacial scales larger than the sizes of particles forming the cluster and smaller than the size of the whole cluster; therefore, to track the fractal geometry in a single aggregate it must be relatively large. However, an ensemble of small aggregates of particles, with the number of particles on the order of only ten or more, can also manifest the fractal geometry statistically, on average, despite the fact that single clusters do not manifest the fractal geometry when considered individually. Thus, the term fractals comprises an ensemble of large aggregates (the ensemble can be small and consist of few, or even one, cluster), or a large ensemble of small aggregates of particles, which statistically show the fractal (scale-invariant) geometry with some interval of sizes.
Enhanced optical response in metal nanocomposites characterized by fractal geometry and thin metallic films containing nanoscale surface features has been intensively studied. R. K. Chang and T. E. Furtak, Ed., Surface Enhanced Raman Scattering (Plenum Pres, NY, 1982); M. Moskovits, Rev. Mod. Phys. 57, 783 (1985); R. W. Boyd, et al., Pure Appl. Opt. 5, 505 (1996); V. M. Shalaev and M. I. Stockman, Sov. Phys. JETP 65, 287 (1987); V. A. Markel, et al., Phys. Rev. B 43, 8183 (1991); V. M. Shalaev, Phys. Reports 272, 61 (1996); V. A. Markel, et al., Phys. Rev. B 53, 2425 (1996); V. M. Shalaev, et al., Phys. Rev. B 53, 2437 (1996); M. I. Stockman, Phys. Rev. Lett. 79, 4562 (1997); S. G. Rautian, et al., JETP Lett. 47, 243 (1988); V. P. Safonov et al., Phys. Rev. Lett 80, 1102 (1998); V. M. Shalaev et al., J. Nonlinear Optical Physics and Materials 7, 131 (1998). Enhancement in the optical response is associated with the excitation of surface plasmons, collective electromagnetic modes whose characteristics are strongly dependent on the geometrical structure of the metallic component of the medium. Collective optical excitations, such as surface plasmons, are often spatially localized in fractals. This localization leads to the presence of nanometer-scale spatial regions of high local electric fields, “hot spots”, and accordingly, to significant enhancement for a variety of optical processes, such as Raman scattering, four-wave mixing, and nonlinear absorption and refraction. In some cases, the local enhancement at a hot spot can be 109 greater than the average enhancement resulting from the fractal itself, averaged over the entire surface of the fractal.
Fractals also have another important property—they are subject to surface enhanced Raman scattering (SERS) by adsorbed molecules. Suitable substrates known to exhibit SERS include colloidal metal particles, vacuum deposited films, single crystals, and matrix isolated metal clusters. O. Silman, et al., J. Phys. Chem. 87, 1014–23 (1983). Also, adsorption of dye molecules, e.g., Rhodamine 6G (R6G), on colloidal Ag or Au is known. P. C. Lee and D. Melsel, J. Phys. Chem. 86, 3391–95 (1982). Once adsorbed onto the colloidal particle, the adsorbed molecules may exhibit strong surface enhanced Raman scattering.
Fractal aggregates of metal nano-sized particles can provide dramatic enhancement for various linear and nonlinear optical responses, including Raman Scattering (RS) and Hyper-Raman Scattering (HRS). This occurs because of localization of optical plasmon excitations within small parts of a fractal aggregate, hot spots, smaller than the size of the fractal and often smaller than the wavelength. When sufficiently concentrated, the large electromagnetic fields in the hot spots can result in very large enhancement of optical responses. The small areas, where the fractal optical excitations are localized, have very different local structures and, therefore, they are characterized by different resonant frequencies. Because of the large variety in local geometries of fractal hot spots, the normal modes of a fractal aggregate cover a huge spectral range, from the near ultra-violet to the far-infrared, leading to giant enhancement of optical responses within this large spectral range. Furthermore, since the dielectric constant of metal is negative and increases in magnitude toward the longer wavelengths, the enhancement for optical processes becomes progressively larger toward infrared (IR) wavelengths.
The various nano-scale areas, where the resonant fractal excitations are localized, can be thought of as a set of different optical “nano-resonators”, each having different resonance frequencies resonating in the visible and IR spectral ranges. These fractal nano-resonators have large resonance quality-factors (Q), representing the local-field enhancement, that increase from the visible to the IR region of the spectrum.
Large enhancement for SERS can also be obtained in compact structures, such as nano-sized spheroids or small chain-like aggregates of particles. However, a compact structure of a given geometry has very few normal modes, for example, one, in a sphere, and three, in a spheroid, and thus provide enhancement at only a few selected frequencies. In contrast, in random fractals, there are always such configurations of particles (nano-resonators) that resonate at any given wavelength. Thus, the inherent properties of random fractals provide localization of optical excitations which become sensitive to the local structures. In addition, the fractals exist as a large variety of resonating local structures, which leads to a very broad enhancement band from the near ultra-violet to the far-infrared region of the spectrum.
An alternative approach for achieving large enhancement of the optical response involves the excitation of morphology-dependent resonances (MDRs) in dielectric microcavities. R. K. Chang and A. J. Campillo, Ed., Optical Processes in Microcavities, World Scientific, Singapore-NewJersey-London-Hong Kong (1996). These resonances, which may have very high quality factors, Q on the order of 105 to 109, result from confinement of the radiation within the microcavity by total internal reflection. Light emitted or scattered in the microcavity may couple to the high-Q MDRs lying within its spectral bandwidth, leading to enhancement of both spontaneous and stimulated optical emissions. For example, enhanced fluorescence emission from a dye-doped cylindrical or spherical microcavity occurs when either the laser pump or the fluorescence, or both, couple to microcavity MDRs. J. F. Owen, Phys. Rev. Lett. 47, 1075 (1981). Moreover, the increased feedback produced by MDRs is sufficient to obtain laser emission from a dye-doped microdroplet under both a continuous wave (CW) and pulsed laser excitation. H. M. Tzeng, et al., Opt. Lett. 9, 499 (1984); A. Biswas, et al., Opt. Lett. 14, 214 (1988). The existence of high-Q microcavity modes is also responsible for numerous stimulated nonlinear effects including stimulated Raman and Rayleigh-wing scattering and four-wave parametric oscillation under moderate intensity CW excitation. M. B. Lin and A. J. Campillo, Phys. Rev. Lett. 73, 2440 (1994).
Optical microcavities are resonators that have at least one dimension, on the order of a single or at most a small integral number of optical wavelengths. See Dodabalapur, et al., U.S. Pat. No. 5,405,710, entitled “Article Comprising Microcavity Light Sources.” The specific geometry of the microcavity and the boundary conditions on the interface of the dielectric-to-air impose selective normal modes on the optical microcavity. Typical microcavities have diameters of 100 microns or less. Such microcavities have shown technological promise for constructing novel light emitting devices. Possible applications of microcavities devices include flat panel displays, optical interconnects, optical fiber communications, and light emitting diode (LED) printing. For example, in a display application, a device may consist of three microcavities, each microcavity emitting in the blue, green, and red regions of the visible spectrum. Further, resonant microcavities have the advantage of emitting light in a highly directional manner as a result of their inherent geometry.
As described briefly above fractal aggregates and resonating microcavities are known to cause large enhancements of optical emissions. The present invention uses the properties of nanoparticles, fractals, and microcavities to enhance optical emissions for a variety of apparatuses and methods. The present invention further combines the properties of these optical enhancement processes by placing nanoparticles and/or fractal aggregates within a high-Q microcavity. Overall, the observed optical enhancement of the invention is multiplicative rather than additive of the two processes. Results demonstrate the unique potential of such devices in the development of ultra-low threshold microlasers, nonlinear-optical devices for photonics, as well as new opportunities of micro-analysis, including spectroscopy of single molecules, quantum wells and nanocrystals.
For purposes of the specification and claims, a semicontinuous metal film, also called a random metal-dielectric film, is a thin film comprising randomly distributed metal particles and their clusters at or near the percolation (conductivity) threshold. The percolation threshold is defined as the metal filling factor pc at which the metal-dielectric film experiences a transition from an insulator to a conductor, with respect to the DC electric current. Semicontinuous metal films can be grown on top of a dielectric or semiconductor substrate. A metal film reaches its percolation threshold where there exists a continuous conducting path between two opposite ends of the film. A metal film developed at or near its percolation threshold is semicontinuous, in contrast to discontinuous films at much lower metal-filling factors and continuous films at much higher metal-filling factors.
Surface-plasmon excitations in a semicontinuous metal film are localized in small nanometer-scale volumes, called hot spots. V. M. Shalaev, Nonlinear Optics Of Random Media: Fractal Composites and Metal-Dielectric Films (Springer Verlag, Berlin, December 1999); A. K. Sarychev and V. M. Shalaev, Physics Reports 335, p. 275 (September 2000); S. Grésillon, et al., Phys. Rev. Lett. 82, p.4520 (May 1999); A. K. Sarychev, et al., Phys. Rev. B 60, p. 16389 (December 1999); V. M. Shalaev, et al., Phys. Rev. B 57, p. 13265 (May 1998); A. K. Sarychev, et al., Phys. Rev. E 59, p. 7239 (June 1999). The electromagnetic energy is concentrated in the hot spots, leading to the local optical intensity that can exceed the intensity of the incident light beam by four to five orders of magnitude, i.e., by a factor up to 100,000. The very intense local fields in the hot spots, with dimensions of approximately 10 nm, result in dramatically enhanced linear and, especially, nonlinear optical responses. While a linear optical response is proportional to light intensity, a nonlinear optical response is scaled with the square, cube or even higher power of light intensity and, therefore experiences a larger enhancement.
A semicontinuous metal film provides enhanced linear and nonlinear optical responses as long as its metal-filling factor p satisfies the condition of |p−pc|≦(εdielectric/|εmetal|) 1/(t+s), where pc is the metal-filling factor at the percolation threshold, εdielectric is the dielectric function (i.e., permittivity) of the dielectric component of the semicontinuous metal film, and εmetal is the dielectric function of the metal component of the film. For a three-dimensionally semicontinuous metal film, t=2.05 and s=0.76 so that the exponent 1/(t+s)=0.356. For a very thin semicontinuous metal film, which can be viewed approximately as two dimensional, t=s=4/3 so that the exponent 1/(t+s)=0.375, which is quite close to the three-dimensional value of 0.356. For the purpose of defining the applicable range of a semicontinuous metal film for enhancing optical responses, the metal-filling factor p of the film should be within a range between pc−(εdielectric/|εmetal|)0.36 and pc+(εdielectric/|εmetal|)0.36.
The following patents are illustrative of the prior art, albeit not disclosing use of semicontinuous metal films: U.S. Pat. No. 6,017,630 discloses forming ultrafine particles on a substrate by directing a slanting high energy irradiating beam against side walls of a plurality of pores in a target material. U.S. Pat. Nos. 5,817,410, 4,448,485, 5,401,569, 5,472,777, and 5,113,473 relate to isolated (i.e., independent) particles. U.S. Pat. Nos. 4,583,818, 6,122,091, 5,991,488, 5,067,788, 6,034,809, and 5,858,799 relate to continuous films. Additionally, periodic arrangements, different from random distribution of metal clusters in semicontinuous metal films, are disclosed in U.S. Pat. Nos. 4,583,818, 4,448,485, and 5,113,473.